Background: Infections are frequent after high dose chemotherapy or total body irradiation followed by hematopoietic cell transplantation (HCT). This is due at least in part to the slow reconstitution of CD4 cells. In adult patients, the T cell repertoire is restricted and counts of phenotypically nave T cells are low for years. This is primarily due to the limited ability of adult patients to regenerate T cells de novo (from hematopoietic stem cells). Hypothesis: Interleukin-7 (IL7) hastens the regeneration of T cells de novo. Research design and Methods: Autologous HCT transplantation of irradiated baboons will be performed, using yellow fluorescent protein (YFP) gene-transduced CD34 cells and green fluorescent protein (GFP) gene-transduced T cells. After transplant, de novo T cell generation in IL7-treated versus placebo-treated animals will be evaluated by 1. enumerating YFP+ T cells, 2. determining functional T cell repertoire through the detection of various antigen-specific T cells after immunization with multiple antigens, 3. enumerating phenotypically nave (e.g., CD45RA high) CD4 T cells, 4. assessing the size and histology of the thymus, including RAG and TdT immunostains, thymocyte subsets and T cell receptor rearrangement excision circle (TREC) levels in thymocytes, and 5. determining TREC levels in peripheral T cells. To rule out the possibility that the expected result of increased YFP+/CD45RA high/TREC+ T cell counts in the blood of the IL7-treated animals is due to IL7-induced shift of stem cell-derived T cells from organs to the blood, YFP+, CD45RA high and TREC+ cells will be enumerated in both the blood and lymphatic tissues. We will also evaluate whether IL7 stimulates peripheral expansion of primate T cells by enumerating GFP+ T cells. Moreover, we will determine the efficacious (de novo regeneration-stimulating) dose of IL7 in primates and potentially toxicity including osteoporosis or a lymphoproliferative disorder. These results will be used for the design of clinical studies.